1. Field of the Invention
The present invention relates to a vibration determination method for determining the type of chatter vibration occurring in a machine tool during machining performed by rotating a tool or a workpiece during machining, and a vibration determination device for executing the vibration determination method.
2. Description of Related Art
In a machine tool performing machining by rotating a rotary shaft, so-called chatter vibration may possibly occur during machining due to improper machining conditions such as depth of cut and rotation speed of the rotary shaft, and the like. When the chatter vibration occurs, finishing accuracy of the machined surface may be reduced and the tool may be broken. Therefore, suppression of the chatter vibration is required.
There are two types of chatter vibration, namely, “regenerative chatter vibration” which is self-induced vibration occurring between a tool and a workpiece, and “forced chatter vibration” in which the machine tool itself becomes a source of vibration. Prior to the present application, the applicant devised a vibration suppression device (Japanese Unexamined Patent Application Publication No. 2008-290186) that discriminates the two types of chatter vibration so that respective countermeasures can be taken for the two types of chatter vibration. In the vibration suppression device described in Japanese Unexamined Patent Application Publication No. 2008-290186, frequency-domain vibrational acceleration is obtained through FFT-analysis on time-domain vibrational acceleration detected by vibration sensors, and the frequency at which the frequency-domain vibrational acceleration becomes maximum is obtained as the chatter frequency fc. Then, k′ number, k number, and phase difference ε are calculated according to formulae (1) to (3) below to determine “forced chatter vibration” when the phase difference ε is close to 0 (0.1 or less, for example), that is, the case k′ number is close to an integer, and to determine “regenerative chatter vibration” otherwise. In other words, when machining is performed at the rotation speed Sa of the rotary shaft using a tool having such vibration characteristics as shown in FIG. 7 for example and if “forced chatter vibration” occurs, as shown in a graph of FIG. 6, a value of an integral multiple of a fundamental frequency (a value close to an integral multiple in actual machining) is detected as the chatter frequency. Therefore, the case the phase difference ε takes a value close to an integral multiple of the fundamental frequency is determined as “forced chatter vibration.”[FORMULAE]k′=60×fc/(Z×S)  (1)k=|k′┘  (2)ε=k′−k  (3)In the formula (1), Z represents the number of tool flutes, and S represents the rotation speed of the rotary shaft in terms of the revolutions per minute. In the formula (2), |x┘ represents a floor function expressing a maximum integer smaller than x (that is, according to the formula (2), an integral portion of k′ number is obtained).
However, when the rotation speed of the rotary shaft is low and if machining is performed using a tool with small number of flutes, as shown in an area a (the area of low rotation speed) in the graph of FIG. 6 for example, the interval of the chatter frequency which is the integral multiple of the fundamental frequency becomes dense. In other words, in such a machining condition, the chatter frequency hardly differs between “forced chatter vibration” and “regenerative chatter vibration”. Therefore, when the chatter vibration is determined using only the maximum value in the frequency-domain vibrational acceleration as described in Japanese Unexamined Patent Application Publication No. 2008-290186, determination accuracy between “forced chatter vibration” and “regenerative chatter vibration” may become poor in the machining condition described above.
Further, with respect to “forced chatter vibration,” in addition to “synchronizing type vibration” in which the chatter frequency takes the values of the integral multiples of the fundamental frequency as described above, there is “natural type vibration” occurring at a frequency in the vicinity of the natural frequency of the machine tool body, tool, workpiece and the like due to friction between the tool and the workpiece and an impact force caused by machining. The chatter frequency of such “natural type vibration” is not stable. That is, when the frequency-domain vibrational acceleration is obtained through FFT-analysis on the time-domain vibrational acceleration, the maximum value thereof appears in the vicinity of the natural frequency of the machine tool body and the tool system, and the frequency that takes the maximum value is not constant. Accordingly, there is also a problem that “natural type vibration” cannot be determined when only the maximum value of the frequency-domain vibrational acceleration is made an object as described in Japanese Unexamined Patent Application Publication No. 2008-290186.
The present invention has been developed under the circumstance, and its object is to provide a vibration determination method and a vibration determination device capable of determining the type of the chatter vibration more accurately than done in the prior art.